Multistage vacuum pumping system

ABSTRACT

A self-regulating vacuum pumping system comprising a first stage positive displacement pump arranged in series with a second stage liquid ring pump. The high volumetric efficiency of the positive displacement pump is substantially improved by the combination compression, cooling and condensing action of the liquid ring pump.

United States Patent Huse 1 Feb. 15, 1972 [54] MULTISTAGE VACUUM PUMPING1,049,894 l/l9l3 Merrill ..4l7/62 SYSTEM 2,971,691 2/1961 Lorenz..4l7/69 [72] Inventor: Henry Huse, 135 South Porter Ave., Wau primwminer Roben w keshfl, 53136 Attorney-Fred Wiviott and Ralph G.Hohenfeldt [22] Filed: Nov. 19, 1969 ABSTRACT [2]] Appl' 877306 Aself-regulating vacuum pumping system comprising a first stage positivedisplacement pump arranged in series with a [52] US. Cl... ..4l7/205,417/243 second stage liquid ring pump. The high volumetric efficiency[51] Int. Cl ..F04b 23/08, F04b 23/00 of the positive displacement pumpis substantially improved [58] Field of Search ..4l7/68, 69, 243, 205,62, 438 by the combination compression, cooling and condensing action ofthe liquid ring pump. [56] References Cited 13 Claims, 3 Drawing FiguresUNITED STATES PATENTS 694,299 2/1902 Ostergren ..417/438 PATENTEDFEB 1 5I972 I HENRY HUSE INVENTOR BY I M ATTORNEY BACKGROUND OF THE INVENTIONIn the application of vacuum pumping apparatus to saturated vapor-gassystems, such as the exhaust from a condenser system in which it isdesired to keep the temperature constant, and to achieve lowest possiblepressure, it becomes increasingly difficult to obtain a deeper vacuumbecause the vapor pressure of the gases is ordinarily a limiting factor.As the partial gas pressure approaches zero, the resultant volume of thegases approaches infinity. Likewise, for a given weight of air or gasthe volume of saturation vapor increases greatly as the partial air orgas pressure is decreased. The vapor pressure limitation is even moreserious as the absolute pressure ap proaches the vapor pressure of theliquid in saturation. No way is presently known to extract the gaswithout removing the vapor. It is therefore necessary to provide anefficient means to extract the noncondensable gases, and to condense andseparate the vapors from the gases.

FIELD OF THE INVENTION The main condensers for steam turbine systemsrequire high vacuum to realize maximum turbine efficiency. Otherapplications where a self-regulating vacuum pumping system is highlydesirable include vacuum dryers, vacuum cookers, vacuum strippers forchemical reactors, sterilizers, autoclaves and rotary vacuum filters.When evacuating condensers, evaporators, dryers, and other systems wherevapor is present it is necessary to remove not only the dry aircomponent but the accompanying vapor component. This imposes a largevolumetric load in the exhauster. Applicants invention is particularlyuseful in such systems, which require the efficient hamdling ofair-vapor mixtures.

DESCRIPTION OF THE PRIOR ART Multistage steam ejectors have been usedfor producing vacuum in the main condensers of turbogenerator systems.However, such systems are difficult to automate, and are limited toextracting air-vapor mixtures on a constant weight basis. Largevariations in the air-vapor volume may upset the jet equilibrium,causing loss of capacity. To avoid this, the apparatus is oversized.Water eductors have been used, but they are even less efficient.

Reciprocating, rotary piston, sliding vane and liquid ring vacuum pumpshave also been used for condenser vacuum systems. Those which requirelubrication cannot normally tolerate condensation during compression.The liquid ring vacuum pump is most desirable, because it is reliable,is a nonlubricated type of pump, and requires little maintenance.However, the liquid ring pump requires a substantial partial airpressure to operate efficiently and to avoid cavitation effects.

In the past, the liquid ring pump has been coupled with an air ejectorto provide a means for raising the vacuum into the desired high vacuumrange. In such a system, the air removal efficiency is low and the powerrequirements are high. For every pound of noncondensable gasesextracted, the system must handle about three pounds of motive air,thereby requiring a substantially oversized liquid ring pump for thesecond stage of the system.

A combination of a first stage liquid ring pump with a second stagecentrifugal pump has also been suggested. This combination, however, isintended primarily to handle large quantities of liquid with someentrained gases and vapors. In this combination, flooding of the liquidring pump is not avoided, and the system merely uses the centrifugalpump to improve the water handling characteristics of the liquid ringpump. Such operation offers no increase in volumetric effi ciency, andthere is no means for avoiding prolonged marginal operation of theliquid ring pump. In the flooded condition, the centrifugal pump is, ineffect, pulling the liquid through the liquid ring pump. The centrifugalpump does not increase the actual volumetric efficiency of the liquidring pump but merely prevents it from overloading and stalling thecommon drive motor. In such an arrangement, the centrifugal pump isnecessarily oversized.

SUMMARY OF THE INVENTION This invention is directed to the combinationof a rotary positive displacement vacuum booster pump as the first stageof a vacuum pumping system and a liquid ring vacuum pump second stage.The system provides high volumetric efficiency over a wide operatingvacuum range with low power requirements. The system is compact,inherently self-regulating, and the-rotary components require nolubrication. since there is no metal-to-metal contact in either thefirst stage, lobe-type positive displacement vacuum booster pump, or inthe second stage liquid ring pump. The rotary positive displacement pumpselected is of normal size for the operating capacity required, and itsvolumetric efficiency is substantially increased by the second stageliquid ring pump. The advantages of both types of pumps are more fullyrealized in the integrated, thermodynamically balanced system providedby the invention.

The two types of pumps provide an ideal combination for series operationand for handling liquid entrained with the pumped fluid, as well asliquid condensed during compression. The first stage pump is capable ofcompressing saturated vapor-gas mixture in a ratio of as much as eightto one. The heated and saturated vapor-gas mixture may then be cooled bya water spray in between the first and second stage pumps. The mixtureis then pulled into the second stage liquid ring pump where thesaturated vapor-gas mixture is further cooled and compressed, withfurther condensing of the vapor component of the mixture tosubstantially increase the capacity of the second stage liquid ringpump. Although, the system finds best use for pumping saturatedvapor-gas mixtures, it can also be used for pumping of dry gases.

A control means is also provided so that the first and second stages ofthe vacuum pumping system are automatically stabilized to insureefficient operation over a full vacuum range. The first stagedisplacement is always kept in balance with second stage capacity bymeans of automatically operated by pass controls.

This invention makes use of the uniquely desirable features of positivedisplacement pumping device as a first stage pump backed by a liquidring centrifugal displacement second stage pump. The thermodynamicrelationship between these pumps provides optimum efficiency, especiallywhen handling saturated fluids at low absolute pressure.

The thermodynamic interaction of the two types of pumping means, thatprovides the outstanding performance and high efficiency that may beachieved with the pumping system described. The first stage rotarypositive displacement pump has exceptionally high volumetric efficiencywhen operating with low pressure differential. The liquid-ring secondstage pump has the ability to act as a condenser, thus reducing thevolume of the vapor component.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE INVENTIONThe vacuum pumping system 1 includes a first stage, rotary positivedisplacement vacuum booster pump 2. The pump 2 is a lobe type, positivedisplacement rotary pump of the Roots type which includes twocounter-rotating rotors 3 and 4 matched to rotate together to trap andpump a fixed volume of a saturated vapor-gas mixture discharged intopump inlet 5 from a condenser 6. Condenser 6 may include an inlet 6-4,which receives exhaust fluid from a system under vacuum. The system mayalso include heat transfer means such as condenser coils 6-2, a drain6-3, and a drain control valve 6-4. An inlet check valve to the vacuumpumping system 1 is also shown. The condenser 6 may, of course, bearranged so that only a gaseous mixture is transferred through the inletcontrol valve 6-5 to the vacuum pumping system 1.

Alternately, the arrangement permits liquid plus gaseous fluids to behandled by the vacuum pumping system 1. The saturated vapor gas mixtureis displaced as it is carried through casing 7 and discharged at outlet8. This pumping action is repeated four times per revolution of eachrotor 3 and 4. The rotors 3 and 4 rotate without contact, but with closeclearances, to attain maximum volumetric efficiency.

The saturated vapor-gas mixture increased in temperature and pressure asit is compressed by the pump 2. The compressed saturated vapor-gasmixture passing through outlet 8 then normally passes through aninterstage cooler 9, which may take the form of a water spray nozzle 10disposed in conduit 11. An interstage bypass conduit 12 branches offfrom the conduit 11 downstream from the pump 2. A check valve 13 may beinstalled in conduit 11 to prevent backflow during shutdown, conduit 12.A second stage liquid ring pump 14 is connected to the pump 2 by conduit11 downstream from the interstage cooler 9.

The liquid ring pump 14 includes an eccentric casing 15, an inlet 16, anoutlet 17 and a rotor assembly 18, shown schematically in FIG. 1. Theoutlet 17 connects through conduit 19 to a separator tank 20. The upperend of the separator tank 20 includes a vent 21 to atmosphere. Anentrainment separator screen 22 is normally disposed across vent 21.

A bypass conduit 23 interconnects the interstage bypass conduit 12 tothe upper portion of the separator tank 20. A pressure regulated checkvalve 24 is disposed in conduit 23 to control direct flow to theseparator tank 20 from the positive displacement pump 2. A recirculationconduit 24 is also connected to the interstage bypass conduit 12. Therecirculation conduit 24 interconnects the outlet 8 of the positivedisplacement pump 2 to its own inlet 5. A pressure relief valve 25 isprovided in conduit 24 to limit the pressure in the recirculation loopto the designed operating pressure range of the pump 2.

The pressure relief valve 25 is adjustable, and is used to control thedifferential pressure across the first stage positive displacement pump2. The relief valve 25 effectively reduces the power requirement for.the system because the power requirements in the first stage (booser)pump 2 is a direct function of pressure differential, and the valve 25keeps the power requirements low by limiting the pressure differentialacross the pump 2. This is particularly important during startup(pullup) when thevacuum pumping system 1 commences operation. The reliefvalve 25 insures that the positive displacement pump will not besubjected to an excessive load, and permits sizing the first stage pumpfor a larger staging ratio.

The separator tank 20 is also provided with a sealing water conduit 26which recirculates sealing water through a heat exchanger 27 providedwith cooling water through cooling water tubes 28. A sealing water feedconduit 29 supplies water to the interstage cooler 9 through branch 30.A second branch 31 may be included to supply water to a second cooler 32which is disposed in suction intake conduit 33 of the vacuum pumpingsystem 1. As shown, the suction intake conduit 33 is connected toexhaust conduit 34 of a condenser 6 in a steam turbine system.

The main components of the system are the first stage positivedisplacement pump 2, the second stage liquid ring pump 14, the separatortank 20 and the heat exchanger 27. The pumps 2 and 14 are driven by anysuitable drive means, such as an electric motor. The pumps 2 and 14 maybe driven by individual motors, or a single motor can be used to driveboth.

The rotors3 and 4 of the first stage positive displacement pump 2 arematched to counter rotate freely with substantially no metal-to-metalcontact, driven by a motor (not shown) connected to the rotor 3. Thesaturated vapor-gas mixture from the condenser 6 is trapped byintermeshing lobes 36 of the rotors 3 and 4, compressed, and thendischarged through outlet 8 into interstage conduit 11. A water spraycooler 37, similar to the interstage cooler 9, injects sealing waterinto the inlet 5 of the pump 2 to seal the clearances between the lobes36, and to aid in cooling the pump 2, which is heated by the risingtemperature of gases being compressed therein.

The second stage liquid ring pump 14 has only one moving part, a vanedrotor 38, which is part of the rotor assembly 18. As can be seen in FIG.2, the vaned rotor 38 which is driven by the motor rotates freely arounda stationary port cylinder 39. The rotor 38 and the port cylinder 39 areconcentric, but the casing 15 has an eccentric volute 40 formed therein.Sufficient sealing water is supplied through interstage cooler 9 to theliquid ring pump 14 to form a liquid ring 41 inside the casing 15conforming to the eccentric contour of the casing 15. As

can be seen in FIG. 2, the port cylinder'39 is provided with an inletport 42 radially inward from the portion of liquid ring 41 as it isreceding away from the port cylinder 39, thereby defining chambers 43between adjacent rotor vanes 44 for receiving the saturated vapor-gasmixture from the inlet 16. As rotation continues, the chambers 43enlarge to a maximum, and then decrease as the rotating liquid ring 41is guided closer to the portcylinder 39 by the eccentric casing 15. Anoutlet port 45 is disposed on the port cylinder 39 radially inward fromthe portion of the liquid ring 41 which is advancing closer to the portcylinder 39. in this manner, the rotor vanes 44, in cooperation with theliquid ring 41, pump the saturated vaporgas mixture through the pump 14,the liquid ring 41 serving both to further compress the vapors andnoncondensable gases, and to cool the mixture to further reduce thevolume by condensing the vapors.

The liquid ring pump 14 discharges through discharge conduit 19 into theseparator tank 20. The discharge conduit 19 normally enters the tank 20below the liquid level 46, and is provided with a perforated endextension 47 to better distribute the now substantially condensed vaporsand noncondensable gases. The gases bubble up to escape through the vent21 to the atmosphere. The separator tank 20 serves both as a reservoirfor seal water for pumps 2 and 14, and as a gas separator for thecompressed gas-condensed vapor mixture discharged from pump 14 as wellas silencer.

The heat exhanger 27 further cools the condensate which is used as sea]waterin the pumps 2 and 14. Raw water, or any other readily availablecooling liquid, may be used as the cooling medium, and is normallypumped through the tubes 28 at a rate to obtain the optimum operatingtemperature.

The thermodynamic interaction of the combination pumping system is amost significant feature of the invention which can be illustrated by aspecific example. A first stage, positive displacement pump is employedwhich is designed to handle 60 lb./hr. dry air plus water to saturate at84 F. (1,260 cubic feet per minute input volume) with input pressure ofL5 in. Hg absolute. The first stage pump is coupled with a second stageliquid ring pump which can handle an input volume of 212 cubic feet perminute at 94 F., and input pressure is 3.5

in. Hg absolute.

For this combination, the first stage pump recorded an outlet pressureof 3.5 in. Hg, outlet temperature was 109 F., and the output volume wasreduced to 438 cubic feet per minute giving a compression ratio in thefirst stage of about 2.33 to l. The partial air pressure at the inlet tothe first stage was 0.325 in. Hg, and the outlet partial air pressurewas 0.979 in. Hg, giving a ratio of 5.8:1 on partial air pressure forthe first stage. The air being handled through the system remainedconstant at l lb. per minute through both stages.

The water vapor volume entered the first stage at a rate of 2.24 lbs.per minute, and left the first stage at a rate of 1.60 lbs. per minuteeffecting a condensation rate of 0.64 lbs. per minute.

The interstage cooler 9, for the example given, cooled the saturatedvapor-gas-liquid mixture from the first stage pump from 109 to 94 F.,reducing the partial vapor pressure from about 2.52 in. Hg to 1.61 in.Hg. The partial air pressure increased from about 0.98 in. Hg to about1.89 in. Hg. The interstage cooling reduced the volume of the flowmixture substantially, from 438 cubic feet per minute, to 212 cubic feetper minute, and the water vapor volume was reduced here from 1.60 lbs.per minute to 0.85 lbs. per minute.

The second stage liquid ring pump 14 then received the saturatedvapor-gas-liquid mixture at a pressure of 3.50 in. Hg (abs) anddischarged at 29.92 in. Hg. The temperature through the pump 14 roseonly 6 F., from 94 to 100 F. The partial vapor pressure increased onlyabout 0.31 in. Hg, from 1.61 in. Hg to 1.92 in. Hg. Partial air pressurerose from 1.89 in. Hg to 28.00 in. Hg, and the volume of the mixture wasreduced substantially from a flow rate of 212 cubic feet per minute to15.1 cubic feet per minute. The most significant effect obtained fromthe second stage liquid ring pump 14 was the condensing ofthe watervapor, which entered the pump 14 at a flow rate of 0.85 lb. per minuteand discharged at 0.043 lb. per minute, a reduction of better than 0.81lb. per minute.

In the example given above, the first stage compression ratio was 2.33to l on total pressure, 5.8 to l on partial air pressure. The stagingratio for the first stage was approximately 6 to 1. Throughout thecompression, the condensing effect acts like a vapor extractor, so thereduction in partial air pressure observed correlated very closely tothe first stage staging ratio.

The second stage compression ratio was approximately 8.5: 1, and partialair pressure was increased at a ratio of 15:1. These latter ratiosillustrate the excellent performance obtained by the combination of theinvention, and the full utilization of the second stage liquid ringpumps ability to handle a substantial amount of water without damage.

The example shows that the compression is obtained under nearlyisothermal conditions, the temperature rise throughout the system beingonly about 16 F. Water vapor is condensed at a rate of about 2.20 lbs.per minute, or approximately 2,400 BTUs per minute thermal energy (56.5h.p.). The relatively low temperature throughout compression simplifiedthe machine design and material selection for fabricating the componentsof the system.

' The first stage positive displacement pump 2 complements and enhancesthe operation of the second stage liquid ring pump 14 for vacuum systemsin which a saturated vapor-gasliquid mixture must be handled. Thepositive displacement pump provides exceptionally high volumetricefficiency when displacing large volumes of gas over low pressuredifferential. The highly efficient condensing effect during compressionof the liquid ring pump enhances the volumetric efficiency of thepositive displacement pump in the overall system. In addition, theliquid ring pump can operate efficiently at compression ratios up to tento one with atmospheric discharge. When properly sized and matched, thetwo dissimilar pumps provide a substantially more economical and anefficient wet vacuum pumping system.

1 claim:

1. A multistage evacuating pumping system, comprising in combination afirst stage pump and a second stage pump each having an inlet andoutlet, said first stage pump comprising a positive displacement pumpand said second stage pump comprising a liquid ring pump, meansproviding direct fluid communication from the outlet of the first stagepump to the inlet of the second stage pump, recirculation conduit meansconnecting the outlet of the first stage pump to the inlet of the firststage pump and valve means 'operatively associated with saidrecirculation conduit and responsive to the pressure difference betweensaid first stage pump outlet and said first stage pump inlet formaintaining the pressure across said first stage pump below apredetermined value.

2. The vacuum pumping system of claim 1, including auxiliary coolingmeans for further reducing the temperature of the vapor-gas-liquidmixture being pumped through the system.

3. The vacuum pumping system of claim 1, including interstage coolingmeans disposed between the first stage, positive displacement pump andthe second stage, liquid ring pump to provide additional cooling liquidmixtureupstream from the inlet to the second stage liquid ring pump.

4. The vacuum pumping system of claim 1, including a separator tank forreceiving the discharged condensed vaporgas-liquid mixture from theoutlet of the second stage liquid ring pump, and a gas vent disposed inthe upper portion of said separator tank for discharging gases from thesystem. 5. The apparatus of claim 1, including a second stage liquidrecirculation loop interconnecting the outlet of the second stage liquidring pump with the inlet thereof to provide sufficient liquid to thesecond stage liquid ring pump for uninterrupted operation of the vacuumpumping system.

6. The apparatus of claim 5, including heat exchanger means disposed insaid liquid recirculation loop for regulating the temperature of theliquid recirculated to the second stage liquid ring pump.

7. The apparatus of claim 1, in which the auxiliary cooling meansincludes a liquid spray injected into the fluid stream downstream fromthe positive displacement pump and upstream from the liquid ring pump.

8. The apparatus of claim 1, including a separator tank disposed incommunication with the outlet of the liquid ring pump said tankproviding means for separating gases from liquids discharged from theliquid ring pump.

9. The apparatus of claim 1, including an auxiliary bypass conduit meanconnected to the outlet of the positive displacement pump andpressure-responsive valve means in said conduit to provide an auxiliaryflow path bypassing said liquid ring pump at times when the quantity offluid pumped through the positive displacement pump exceeds the capacityof the liquid ring pump.

10. The apparatus set forth in claim 1 and including a first stageliquid recirculation loop connected to the second stage liquidrecirculation loop and to the inlet of the first stage pump, said firstand second stage liquid recirculation loops including valve means forcontrolling flow therethrough, whereby the quantity of recirculatedcooling liquid mixture is regulated to maintain optimum thermodynamicconditions in a vacuum pumping system.

11. A multistage evacuating pumping system for pumping a saturatedvapor-gas-liquid mixture comprising a combination, a first stage,positive displacement pump having an inlet and outlet, a second stageliquid ring pump having an inlet and outlet, and means providing directfiuid communication from the outlet of the first stage, positivedisplacement pump to the inlet of the second stage liquid ring pump, asecond stage liquid recirculation loop interconnecting the outlet of thesecond stage liquid ring pump with the inlet thereof to providesufficient liquid to the second stage liquid ring pump for uninterruptedoperation of the vacuum pumping system, the first stage recirculationconduit for the first stage for recirculating the saturated vapor-gasmixture from the outlet of the first stage pump to the inlet of thefirst stage positive displacement pump, and a first stage liquidrecirculation loop connected to the second stage liquid recirculationloop and to the inlet to the first stage pump, said first and secondstage liquid recirculation loops including valve means for controllingflow therethrough, whereby the quantity of recirculated cooling liquidmixture is regulated to maintain the optimum thermodynamic conditions inthe vacuum pumping system.

12. The apparatus of claim 11, including a bypass discharge conduitmeans interconnecting the first stage recirculating conduit to adischarge outlet bypassing the second stage liquid ring pump to providean auxiliary discharge outlet for said first stage positive displacementduring startup of the system and periods of operation when the capacityof the second stage liquid ring pump is temporarily exceeded.

13. The apparatus of claim 12, including valve means in the bypassdischarge conduit for regulating the discharge therethrough.

1. A multistage evacuating pumping system, comprising in combination afirst stage pump and a second stage pump each having an inlet andoutlet, said first stage pump comprising a positive displacement pumpand said second stage pump comprising a liquid ring pump, meansproviding direct fluid communication from the outlet of the first stagepump to the inlet of the second stage pump, recirculation conduit meansconnecting the outlet of the first stage pump to the inlet of the firststage pump and valve means operatively associated with saidrecirculation conduit and responsive to the pressure difference betweensaid first stage pump outlet and said first stage pump inlet formaintaining the pressure across said first stage pump below apredetermined value.
 2. The vacuum pumping system of claim 1, includingauxiliary cooling means for further reducing the temperature of thevapor-gas-liquid mixture being pumped through the system.
 3. The vacuumpumping system of claim 1, including interstage cooling means disposedbetween the first stage, positive displacement pump and the secondstage, liquid ring pump to provide additional cooling liquid mixtureupstream from the inlet to the second stage liquid ring pump.
 4. Thevacuum pumping system of claim 1, including a separator tank forreceiving the discharged condensed vapor-gas-liquid mixture from theoutlet of the second stage liquid ring pump, and a gas vent disposed inthe upper portion of said separator tank for discharging gases from thesystem.
 5. The apparatus of claim 1, including a second stage liquidrecirculation loop interconnecting the outlet of the second stage liquidring pump with the inlet thereof to provide sufficient liquid to thesecond stage liquid ring pump for uninterrupted operation of the vacuumpumping system.
 6. The apparatus of claim 5, including heat exchangermeans disposed in said liquid recirculation loop for regulating thetemperature of the liquid recirculated to the second stage liquid ringpump.
 7. The apparatus of claim 1, in which the auxiliary cooling meansincludes a liquid spray injected into the fluid stream downstream fromthe positive displacement pump and upstream from the liquid ring pump.8. The apparatus of claim 1, including a separator tank disposed incommunication with the outlet of the liquid ring pump said tankproviding means for separating gases from liquids discharged from theliquid ring pump.
 9. The apparatus of claim 1, including an auxiliarybypass conduit mean connected to the outlet of the positive displacementpump and pressure-responsive valve means in said conduit to provide anauxiliary flow path bypassing said liquid ring pump at times when thequantity of fluid pumped through the positive displacement pump exceedsthe capacity of the liquid ring pump.
 10. The apparatus set forth inclaim 1 and including a first stage liquid recirculation loop connectedto the second stage liquid recirculation loop and to the inlet of thefirst stage pump, said first and second stage liquid recirculation loopsincluding valve means for controlling flow therethrough, whereby thequantity of recirculated cooling liquid mixture is regulated to maintainoptimum thermodynamic conditIons in a vacuum pumping system.
 11. Amultistage evacuating pumping system for pumping a saturatedvapor-gas-liquid mixture comprising a combination, a first stage,positive displacement pump having an inlet and outlet, a second stageliquid ring pump having an inlet and outlet, and means providing directfluid communication from the outlet of the first stage, positivedisplacement pump to the inlet of the second stage liquid ring pump, asecond stage liquid recirculation loop interconnecting the outlet of thesecond stage liquid ring pump with the inlet thereof to providesufficient liquid to the second stage liquid ring pump for uninterruptedoperation of the vacuum pumping system, the first stage recirculationconduit for the first stage for recirculating the saturated vapor-gasmixture from the outlet of the first stage pump to the inlet of thefirst stage positive displacement pump, and a first stage liquidrecirculation loop connected to the second stage liquid recirculationloop and to the inlet to the first stage pump, said first and secondstage liquid recirculation loops including valve means for controllingflow therethrough, whereby the quantity of recirculated cooling liquidmixture is regulated to maintain the optimum thermodynamic conditions inthe vacuum pumping system.
 12. The apparatus of claim 11, including abypass discharge conduit means interconnecting the first stagerecirculating conduit to a discharge outlet bypassing the second stageliquid ring pump to provide an auxiliary discharge outlet for said firststage positive displacement during startup of the system and periods ofoperation when the capacity of the second stage liquid ring pump istemporarily exceeded.
 13. The apparatus of claim 12, including valvemeans in the bypass discharge conduit for regulating the dischargetherethrough.